Portable radiotelephones for use in cellular networks are sometimes equipped with touch sensitive displays, or touch screens. The concept behind a touch screen is to facilitate the selection of a symbol by the simple act of pointing to, and touching a displayed symbol, rather than typing a particular keystroke on a keyboard.
Because of the relatively small size of a telephone display that is a result of the tendency to miniaturize portable equipment, there is generally not enough room on the face of a display to: (a) depict all the symbols available to the user for the operation of the telephone; and/or (b) address each symbol by human fingertip means.
As an example of the former, all symbols such as numerals, alphanumerics, menu items, special icons, etc. that are related to telephone functions that may be available for use on a portable telephone cannot generally be allocated enough display pixels on the touch sensitive part of the display to visually distinguish and uniquely define each symbol. One approach of the prior art to accommodate large numbers of symbols on a single display was to allocate a fixed, small number of display pixels for each telephone function on a touch sensitive screen. The difficulty with this approach is that sometimes the number of pixels allocated is insufficient to allow the easy identification of the character or function to be activated by touching the specific area. That is, with a limited number of pixels it may be difficult to create a sufficient number of visual features of a symbol to allow its unambiguous identification.
Another problem stems from the small amount of display area allocated to each symbol, legend, and/or related function on a touch sensitive screen for its activation. This limitation arises because the total area necessary to accommodate sufficient space to accommodate the tip of the human finger, for each symbol, would typically exceed the total area of the display itself. When the symbols are made smaller and spaced closer together, the user's fingertip will typically cover one or more adjacent symbols.
With fingertip usage the displayed symbols or touch areas are required to be relatively large (e.g., 7 mm.times.7 mm for index finger use with a 2% error rate). In the past this has forced the use of a stylus, instead of the more natural finger, to achieve acceptable error rates.
That is, using the small area allocated in a touch sensitive display for a multitude of symbols cannot readily accommodate the dimensions of a human fingertip, generally requiring the user to operate the interface using a pointing device, such as a stylus, pencil, pen, etc., that must be placed precisely over the desired character or function to be used to uniquely identify the desired input to the telephone. At times, the precision required in placing the stylus may challenge the coordination skills of the user, especially in a moving train, plane, or automobile where the motion of the vehicle makes it difficult to control the relative position of the stylus with respect to the touch screen. Furthermore, the use of a stylus may be objectionable to some users, and in some situations may present a safety issue.
The foregoing problems will also arise if one were to employ some means other than a touch sensitive display to provide a tactile, non-keypad input to a mobile telephone, especially if a small display device is used.